One Best Hike: Yosemite's Half Dome. Rick Deutsch

of the hike, nothing will replace seeing these majestic sights yourself. I also include many historical vignettes of events that occurred during the evolution of Yosemite. Finally, all opinions and suggestions are mine. Others may approach these topics differently or disagree with me. Pick out what you like and make informed decisions.

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      Geology of Yosemite and Half Dome

      When we try to pick out anything by itself, we find it hitched to everything else in the Universe.

      —John Muir

How Yosemite Was Formed

      Yosemite’s grandeur began about 500 million years ago, when the entire region was an ancient seabed. Since the beginning, the plates that make up the outer portion of the Earth have been moving—coming together and breaking apart. Witness how nicely eastern South America fits into western Africa. About 100 million years ago, the plate west of today’s California, called the Pacific plate, dove eastward under the North American plate, causing the uplift of the region. The mountains still rise today at a rate of 1 foot every 1,000 years. Deep below the ground, constant Earth movement and the effects of pressure and heat created magma, which gradually rose toward the surface. When the magma found boundaries or rifts between the surface plates (subduction zones), it rose up to erupt as volcanoes. This is a phenomenon that happens regularly on Earth. However, sometimes the hot 2,000-degree magma has no route to the surface. It rises but then is forced to lie a few miles below the surface and slowly cool under intense pressure. It then crystallizes into a pluton of granite. When many of these plutons come together, they become batholiths. There are many types of granite, but most consist of a chemical mixture of hornblende, mica, feldspar, quartz, and other minerals. Its classic look is a salt-and-pepper gray, but the color can vary. It can have different patterns and lines due to intrusions during its formation.

      In California, as the Pacific plate slowly crawled under the North American plate, the subduction zone got extremely hot and melted some of the plate, along with some of the overlying crust. Because of the temperature difference with the rocks around it, the magma rose and formed the Sierra granite directly from this subducting slab. This huge mass would later become the underlay of the Sierra Nevada mountains. Most of Yosemite’s granites were formed 85–150 million years ago. Add to the recipe the millions of years of erosion caused by snow, rain, and wind, and gradually the batholiths were exposed. One result was the 80-by-400-mile granite bedrock of the Sierra Nevada.

      The best reader-friendly book on Yosemite’s formation is Geology Underfoot in Yosemite National Park. It was cowritten by Allen F. Glazner, professor of geological sciences at the University of North Carolina, and ranger Greg Stock, the first-ever park geologist at Yosemite. The advances in technology, such as optical remote sensing, high-resolution photography, seismic monitoring, and rockfall runout simulations, are now part of the park’s tools to study rockfalls.

      From about 10–3 million years ago, the Merced and Tuolumne rivers carved down deep into the land with greater intensity. The uplifted land gave more gravitational power to the river. The eroded earth gradually found its way into the Central Valley.

      Uplift and river erosion

      Then, from about 3 million to 18,000 years ago, global cooling resulted in three major glacial periods. Glaciers are simply large masses of snow and ice that do not melt. The snow and ice filled in valleys and accumulated thousands of feet high. Gravity allowed them to slowly advance. As the lower layer of ice picked up rocks, it carved the path of the glacier. The representation below reflects geologists’ belief that Half Dome and many other high peaks were above the glaciers. We will discuss the geologic formation of Half Dome in the next section.

      Early glaciation

      Recent glaciation

      The last major glacier retreated from Yosemite about 18,000 years ago. (We still have two glaciers: Mount Lyell and Mount Maclure.) The classic U shape of glacially carved valleys is not readily evident in Yosemite Valley because research has shown that the bedrock lies about 2,000 feet below today’s roads. How did that happen? During its time, the last glacier slowly covered the area between El Capitan and Bridalveil Fall. As it dug deep, stone and debris were deposited in front of it. When it finally melted and retreated back to the east, the terminal moraine created an earthen dam. The dam stopped the waters from the melting ice and created a lake that geologists call Yosemite Lake. By 10,000 years ago, the lake had filled with sediment, disguising the U shape. Scientists believe the U shape is still there—it’s just 2,000 feet below the surface.

      Melting and lake formation

      Present day Yosemite

      Yosemite is undergoing change even today. Being relatively young geologically, the area is still settling in. Earthquakes have been reported since records were kept in the mid-1800s. Rockfalls are very common at Yosemite. Since 1857, more than 500 rockfalls in Yosemite Valley have killed 14 people and injured more than 60, more than at any other national park. They are the result of quakes and a process of exfoliation, that is, large sheets of granite peeling off in vertical patterns similar to an onion skin. Rockfalls also occur when rainwater gets into cracks and freezes. When water freezes, it expands. The small but constant expansion may cause the granite to open slightly. Lichens can grow here, and when they die, they provide a soil structure in which larger plants can germinate. In time, the growing plant exerts forces that can lead to a flaking off. Given enough time, these processes can take their toll.

      In the 1980s a major rockfall occurred off the Three Brothers formation. In 1996 and again in 1999, enormous sheets of granite dropped near Happy Isles; each event killed one person. The impact produced massive shock waves that literally snapped thousands of trees. The 1996 rockfall was comprised of two slabs (totaling 80,000 tons) bigger than two football fields and three stories tall. They free-fell 1,800 feet and had reached a velocity of 270 miles per hour when they impacted. In 2008 a rockfall off the Glacier Point wall behind Curry Village took more than 200 tent cabins and wooden cabins out of service. You can still see the talus fields from this fall. In 2009 a rockfall even larger than the 1996 Happy Isles fall occurred at Ahwiyah Point near the eastern side of Half Dome’s face. An estimated 115 tons of rock crashed down onto the southern portion of the Mirror Lake Loop Trail. When you are on top of Half Dome, you can carefully crawl out to the edge and look down at the fresh white stone talus field at Ahwiyah Point.

      Flooding has also changed the landscape of the park. The Merced River flood of 1997 set records, and the high water marks are still visible. It caused $178 million in damage in Yosemite Valley. As a result, the valley was closed for more than three months, and vast changes were made to the area’s management plan. Today numerous structures have been relocated, roads have been rerouted out of the flood plain, and nearly 50% of campsites have been removed.

      In 2006 spring rains created a huge rockslide southwest of El Portal that dropped tons of earth from Ferguson Ridge onto CA 140, closing that road for part of the year. Today travel is only possible via two one-way Bailey bridges, which take traffic across the Merced and onto the remnants of the old Yosemite Railroad track, then back 0.25 mile later. The train ceased operation in 1945, and the track was sold off for scrap.